Inverter control mode switching method in multi-inverter accessed distribution network system

Abstract

The application discloses a kind of multi-inverter access distribution network system inverter control mode switching method, including obtaining system parameter;Equivalent output admittance of each inverter is obtained respectively;The full admittance form equivalent circuit model of system is calculated;Phase value is calculated and determined;Normalized phase angle stability margin is calculated and determined, and according to the determination result, the conversion of inverter control mode is carried out;The above steps are repeated in real time to complete inverter control mode switching.The application does not need to add additional equipment, can guarantee new energy to maintain grid-connected power generation, can realize that distribution network system has sufficient stability margin and minimum node voltage deviation, meets the stability requirement of distribution network system, improves the voltage quality of distribution network system;And the application has high reliability, good applicability and good control effect.

Description

Technical Field

[0001] This invention belongs to the field of power electronics, specifically relating to a method for switching inverter control modes in a distribution network system with multiple inverters connected. Background Technology

[0002] With economic and technological development and the improvement of people's living standards, electricity has become an indispensable secondary energy source in people's production and daily life, bringing endless convenience. Therefore, ensuring a stable and reliable supply of electricity has become one of the most important tasks of the power system.

[0003] In recent years, renewable energy sources such as photovoltaics have developed rapidly, and the scale of grid-connected power generation in distribution networks has been expanding year by year. Photovoltaics typically generate electricity via inverters (VSCs) using a grid-connected control mode, offering advantages such as fast control response, better maximum power output, and improved economic efficiency. However, grid-connected photovoltaics cannot provide voltage and frequency support for the distribution network. As the penetration rate of photovoltaic grid connection increases, the stability margin of the distribution network decreases, easily leading to problems such as harmonic resonance, voltage exceeding limits, and degraded power quality in weak grid conditions.

[0004] In order to enable new energy grid-connected equipment to provide voltage and frequency support for the distribution network system, researchers at home and abroad have proposed grid control modes such as vertical control and virtual synchronous generator control (VSG). These technical solutions can provide support for grid voltage and frequency, and have better grid friendliness and security, but they lack current control capabilities and have problems with voltage short circuit overcurrent.

[0005] Because the distribution network status is constantly changing, renewable energy grid-connected equipment needs to select the corresponding control mode based on the grid stability margin to ensure that the voltage, frequency, and other parameters of the grid-connected system do not exceed limits, thus ensuring the safe and stable operation of the distribution network system. Therefore, inverter control mode switching schemes have become an urgent research issue.

[0006] To address this issue, current research solutions both domestically and internationally mainly focus on grid-connected / off-grid switching methods. However, this method is not suitable for application scenarios where grid-connected power generation is maintained continuously. Another switching method is for each inverter to switch modes independently based on the short-circuit ratio at the grid connection point. However, this type of solution can lead to instability in the grid-connected system when inverters with close electrical distances switch to grid-connected mode simultaneously. Therefore, it is not suitable for distribution network systems with multiple inverters connected. Summary of the Invention

[0007] The purpose of this invention is to provide a method for switching inverter control modes in a distribution network system with multiple inverters that has high reliability, good applicability, and good control effect.

[0008] The inverter control mode switching method in a multi-inverter connected distribution network system provided by this invention includes the following steps:

[0009] S1. Obtain the system parameters of the target distribution network system;

[0010] S2. Based on the system parameters obtained in step S1, establish the equivalent output admittance of each grid-connected control mode inverter and each grid-connected control mode inverter respectively;

[0011] S3. Based on the admittance data obtained in step S2, calculate the equivalent circuit model of the target distribution network system in the form of full admittance.

[0012] S4. Calculate the determination phase value between the target distribution network system and the circuit model obtained in step S3;

[0013] S5. Based on the determined phase value obtained in step S4, calculate the normalized phase angle stability margin of the grid-connected system;

[0014] S6. Determine the normalized phase angle stability margin of the grid-connected system obtained in step S5:

[0015] If the normalized phase angle stability margin is less than the set first threshold, then select the optimal inverter among the grid-following control mode inverters and convert it to a grid-connected control mode inverter.

[0016] If the normalized phase angle stability margin is greater than or equal to the first threshold and less than or equal to the second threshold, then the current control mode of all inverters remains unchanged.

[0017] If the normalized phase angle stability margin is greater than the set second threshold, then select the optimal inverter among the grid-connected control mode inverters and convert it to a grid-connected control mode inverter.

[0018] S7. Repeat steps S2 to S6 in real time to complete the inverter control mode switching in a distribution network system with multiple inverters connected.

[0019] Step S1, which involves obtaining the system parameters of the target distribution network system, specifically includes the following steps:

[0020] The target distribution network system includes grid power supply, three-phase load, N grid-connected control mode inverters and M grid-connected control mode inverters; N and M are both natural numbers;

[0021] The acquired system parameters include grid admittance Y. g .

[0022] Step S2, which involves establishing the equivalent output admittance of each grid-connected control mode inverter and each grid-connected control mode inverter based on the system parameters obtained in step S1, specifically includes the following steps:

[0023] Using a harmonic linearized impedance modeling method, the equivalent output admittance Y of N grid-fed control mode inverters is established. eq_1 ,Y eq_2 ,...,Y eq_N ;Y eq_i Let be the equivalent output admittance of the i-th grid-controlled inverter;

[0024] Using a harmonic linearized impedance modeling method, the equivalent output admittance Y of M grid-controlled inverters is established. cq_1 ,Y cq_2 ,...,Y cq_M ;Y cq_j Let be the equivalent output admittance of the j-th grid-controlled inverter.

[0025] Step S3 involves calculating the equivalent circuit model of the target distribution network system in the form of full admittance, which specifically includes the following steps:

[0026] Based on the equivalent output admittance of each inverter obtained in step S2, the equivalent circuit model of the target distribution network system in the form of full admittance is obtained. The equivalent output admittance of each inverter is simplified to the output admittance Y using the following formula. eq :

[0027]

[0028] In the formula Y eq_i Y is the equivalent output admittance of the i-th grid-controlled inverter; cq_j Let be the equivalent output admittance of the j-th grid-controlled inverter.

[0029] Step S4 involves calculating the phase value between the target distribution network system and the circuit model obtained in step S3, specifically including the following steps:

[0030] Plot the output admittance Y eq With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve;

[0031] Based on the plotted curve, obtain Y. eq and Y g The intersection frequency corresponding to the intersection point of the amplitude-frequency response curve;

[0032] In Y eq and Y g On the phase frequency response curve, obtain the output admittance phase value θ1(Y) corresponding to the intersection frequency. eq ) and grid admittance phase value θ1(Y g ).

[0033] Step S5, which involves calculating the normalized phase angle stability margin of the grid-connected system based on the determined phase value obtained in step S4, specifically includes the following steps:

[0034] The normalized phase angle stability margin of the grid-connected system is calculated using the following formula:

[0035]

[0036] In the formula, PM1 is the normalized phase angle stability margin of the grid-connected system.

[0037] If the normalized phase angle stability margin is less than a set first threshold, step S6 involves selecting the optimal inverter from the grid-connected control mode inverters and converting it to a grid-connected control mode inverter. This specifically includes the following steps:

[0038] If the normalized phase angle stability margin is less than the set first threshold, then the following steps A to G are used to calculate the stability margin for each grid-connected control mode inverter in sequence:

[0039] A. Using the impedance modeling method with harmonic linearization, the equivalent output admittance Y when the k-th grid-controlled inverter switches to grid-connected control mode independently is calculated. cq_k ;

[0040] B. Establish an equivalent circuit model in full admittance form after mode switching, and use the following formula to convert the equivalent output admittance of all inverters into the second output admittance Y. cq :

[0041]

[0042] C. Plot the second output admittance Y cq With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve; based on the plotted curves, obtain the Y... cq and Y g The second intersection frequency corresponding to the intersection point of the amplitude-frequency response curves; in Y cq and Y g On the phase frequency response curve, obtain the second output admittance phase value θ2(Y) corresponding to the second intersection frequency. cq ) and the second grid admittance phase value θ2(Y g );

[0043] D. Calculate the second normalized phase angle stability margin of the grid-connected system at this time:

[0044]

[0045] In the formula, PM2 is the second normalized phase angle stability margin of the grid-connected system;

[0046] E. By calculating the power flow, the voltage U of each node in the target distribution network system at this time is obtained. x ;

[0047] F. Calculate the average node voltage deviation ΔU of the target distribution network system at this time:

[0048]

[0049] In the formula, n is the number of nodes in the target distribution network system; U x_ref The nominal voltage of the node;

[0050] G. Calculate the evaluation index Q for the grid control mode switching of the kth grid-controlled inverter. k For Q k =k1*PM2-k2*ΔU; where k1 and k2 are both set weighting coefficients;

[0051] Finally, the inverter with the highest grid-following control mode evaluation index was selected, and the inverter was switched to grid-connected control mode and put into operation.

[0052] If the normalized phase angle stability margin is greater than the set second threshold, then in step S6, an optimal inverter is selected from the grid-controlled inverters and converted to a grid-following control mode inverter. This specifically includes the following steps:

[0053] If the normalized phase angle stability margin is greater than the set second threshold, then steps a to g are performed sequentially to calculate the stability margin for each grid-controlled inverter:

[0054] a. Using the impedance modeling method with harmonic linearization, the equivalent output admittance Y when the k-th grid-controlled inverter switches to grid-following control mode independently is calculated. eq_k ;

[0055] b. Establish an equivalent circuit model in full admittance form after mode switching, and use the following formula to convert the equivalent output admittance of all inverters into the third output admittance Y. eq2 :

[0056]

[0057] c. Plot the third output admittance Y eq2 With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve; based on the plotted curves, obtain the Y... eq2 and Y g The frequency of the third intersection point corresponding to the intersection of the amplitude-frequency response curves; in Y eq2 and Y gOn the phase frequency response curve, obtain the third output admittance phase value θ3(Y) corresponding to the third intersection frequency. eq2 ) and the second grid admittance phase value θ3(Y g );

[0058] d. Calculate the third normalized phase angle stability margin of the grid-connected system at this time:

[0059]

[0060] In the formula, PM3 is the third normalized phase angle stability margin of the grid-connected system;

[0061] e. Through power flow calculation, obtain the voltage U of each node in the target distribution network system at this time. x ;

[0062] f. Calculate the average node voltage deviation ΔU of the target distribution network system at this time:

[0063]

[0064] In the formula, n is the number of nodes in the target distribution network system; U x_ref The nominal voltage of the node;

[0065] g. Calculate the grid-following control mode switching evaluation index Q' of the k-th grid-controlled inverter. k For Q' k =k3*PM3-k4*ΔU; where k3 and k4 are both set weighting coefficients;

[0066] Finally, the inverter with the highest grid-following control mode switching evaluation index was selected, and the inverter was switched to grid-following control mode and put into operation.

[0067] The inverter control mode switching method provided by this invention in a multi-inverter connected distribution network system does not require the addition of extra equipment, can ensure that new energy sources continue to generate electricity on the grid, can achieve sufficient stability margin and minimum node voltage deviation in the distribution network system, meet the stability requirements of the distribution network system, and improve the voltage quality of the distribution network system; moreover, this invention has high reliability, good applicability and good control effect. Attached Figure Description

[0068] Figure 1 This is a schematic diagram of the method flow of the present invention.

[0069] Figure 2 This is a system topology diagram of an embodiment of the present invention.

[0070] Figure 3 This is a schematic diagram of the grid voltage amplitude drop waveform of each VSC before mode switching in an embodiment of the present invention.

[0071] Figure 4 This is a schematic diagram of the voltage drop waveform of each VSC after mode switching in an embodiment of the present invention. Detailed Implementation

[0072] like Figure 1 The diagram shown is a flowchart of the method of the present invention: The inverter control mode switching method in a multi-inverter connected distribution network system provided by the present invention includes the following steps:

[0073] S1. Obtain the system parameters of the target distribution network system; specifically, this includes the following steps:

[0074] The target distribution network system includes grid power supply, three-phase load, N grid-connected control mode inverters and M grid-connected control mode inverters; N and M are both natural numbers;

[0075] The acquired system parameters include grid admittance Y. g ;

[0076] S2. Based on the system parameters obtained in step S1, establish the equivalent output admittance of each grid-connected control mode inverter and each grid-connected control mode inverter; specifically including the following steps:

[0077] Using a harmonic linearized impedance modeling method, the equivalent output admittance Y of N grid-fed control mode inverters is established. eq_1 ,Y eq_2 ,...,Y eq_N ;Y eq_i Let be the equivalent output admittance of the i-th grid-controlled inverter;

[0078] Using a harmonic linearized impedance modeling method, the equivalent output admittance Y of M grid-controlled inverters is established. cq_1 ,Y cq_2 ,...,Y cq_M ;Y cq_j Let J be the equivalent output admittance of the j-th grid-controlled inverter.

[0079] S3. Based on the admittance data obtained in step S2, calculate the equivalent circuit model of the target distribution network system in the form of full admittance; specifically including the following steps:

[0080] Based on the equivalent output admittance of each inverter obtained in step S2, the equivalent circuit model of the target distribution network system in the form of full admittance is obtained. The equivalent output admittance of each inverter is simplified to the output admittance Y using the following formula. eq :

[0081]

[0082] In the formula Y eq_iY is the equivalent output admittance of the i-th grid-controlled inverter; cq_j Let J be the equivalent output admittance of the j-th grid-controlled inverter.

[0083] S4. Calculate the determination phase value between the target distribution network system and the circuit model obtained in step S3; specifically, this includes the following steps:

[0084] Plot the output admittance Y eq With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve;

[0085] Based on the plotted curve, obtain Y. eq and Y g The intersection frequency corresponding to the intersection point of the amplitude-frequency response curve;

[0086] In Y eq and Y g On the phase frequency response curve, obtain the output admittance phase value θ1(Y) corresponding to the intersection frequency. eq ) and grid admittance phase value θ1(Y g );

[0087] S5. Based on the determined phase value obtained in step S4, calculate the normalized phase angle stability margin of the grid-connected system; specifically including the following steps:

[0088] The normalized phase angle stability margin of the grid-connected system is calculated using the following formula:

[0089]

[0090] In the formula, PM1 is the normalized phase angle stability margin of the grid-connected system;

[0091] S6. Determine the normalized phase angle stability margin of the grid-connected system obtained in step S5:

[0092] If the normalized phase angle stability margin is less than the set first threshold (preferred option: PM1 < 1), then select the optimal inverter among the grid-following control mode inverters and convert it to a grid-connected control mode inverter; specifically, the following steps are included:

[0093] If the normalized phase angle stability margin is less than the set first threshold, then the following steps A to G are used to calculate the stability margin for each grid-connected control mode inverter in sequence:

[0094] A. Using the impedance modeling method with harmonic linearization, the equivalent output admittance Y when the k-th grid-controlled inverter switches to grid-connected control mode independently is calculated. cq_k ;

[0095] B. Establish an equivalent circuit model in full admittance form after mode switching, and use the following formula to convert the equivalent output admittance of all inverters into the second output admittance Y. cq :

[0096]

[0097] C. Plot the second output admittance Y cq With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve; based on the plotted curves, obtain the Y... cq and Y g The second intersection frequency corresponding to the intersection point of the amplitude-frequency response curves; in Y cq and Y g On the phase frequency response curve, obtain the second output admittance phase value θ2(Y) corresponding to the second intersection frequency. cq ) and the second grid admittance phase value θ2(Y g );

[0098] D. Calculate the second normalized phase angle stability margin of the grid-connected system at this time:

[0099]

[0100] In the formula, PM2 is the second normalized phase angle stability margin of the grid-connected system;

[0101] E. By calculating the power flow, the voltage U of each node in the target distribution network system at this time is obtained. x ;

[0102] F. Calculate the average node voltage deviation ΔU of the target distribution network system at this time:

[0103]

[0104] In the formula, n is the number of nodes in the target distribution network system; U x_ref The nominal voltage of the node;

[0105] G. Calculate the evaluation index Q for the grid control mode switching of the kth grid-controlled inverter. k For Q k =k1*PM2-k2*ΔU; where k1 and k2 are both set weight coefficients, and the preferred scheme is: k1=k2=0.5;

[0106] Finally, the inverter with the highest grid-following control mode switching evaluation index was selected, and the inverter was switched to grid-following control mode and put into operation.

[0107] If the normalized phase angle stability margin is greater than or equal to the first threshold and less than or equal to the second threshold (preferred option: 1≤PM1≤2), then the current control mode of all inverters remains unchanged.

[0108] If the normalized phase angle stability margin is greater than the set second threshold (preferred option: PM1 > 2), then select the optimal inverter from the grid-connected control mode inverters and convert it to a grid-following control mode inverter; specifically, the following steps are included:

[0109] If the normalized phase angle stability margin is greater than the set second threshold, then steps a to g are performed sequentially to calculate the stability margin for each grid-controlled inverter:

[0110] a. Using the impedance modeling method with harmonic linearization, the equivalent output admittance Y when the k-th grid-controlled inverter switches to grid-following control mode independently is calculated. eq_k ;

[0111] b. Establish an equivalent circuit model in full admittance form after mode switching, and use the following formula to convert the equivalent output admittance of all inverters into the third output admittance Y. eq2 :

[0112]

[0113] c. Plot the third output admittance Y eq2 With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve; based on the plotted curves, obtain the Y... eq2 and Y g The frequency of the third intersection point corresponding to the intersection of the amplitude-frequency response curves; in Y eq2 and Y g On the phase frequency response curve, obtain the third output admittance phase value θ3(Y) corresponding to the third intersection frequency. eq2 ) and the second grid admittance phase value θ3(Y g );

[0114] d. Calculate the third normalized phase angle stability margin of the grid-connected system at this time:

[0115]

[0116] In the formula, PM3 is the third normalized phase angle stability margin of the grid-connected system;

[0117] e. Through power flow calculation, obtain the voltage U of each node in the target distribution network system at this time. x ;

[0118] f. Calculate the average node voltage deviation ΔU of the target distribution network system at this time:

[0119]

[0120] In the formula, n is the number of nodes in the target distribution network system; U x_ref The nominal voltage of the node;

[0121] g. Calculate the grid-following control mode switching evaluation index Q' of the k-th grid-controlled inverter. k For Q' k =k3*PM3-k4*ΔU; where k3 and k4 are both set weight coefficients, and the preferred scheme is k3=k4=0.5;

[0122] Finally, the inverter with the highest grid-following control mode switching evaluation index was selected, and the inverter was switched to grid-following control mode and put into operation.

[0123] S7. Repeat steps S2 to S6 in real time to complete the inverter control mode switching in a distribution network system with multiple inverters connected.

[0124] The method of the present invention will be further described below with reference to an embodiment:

[0125] like Figure 2 The diagram shows the system topology of the target distribution network system according to an embodiment of the present invention. The distribution network system with multiple inverters consists of three VSCs (VSC1, VSC2 and VSC3), grid power supply and three-phase loads. VSC1, VSC2 and VSC3 are initially controlled by the grid, and all three can switch to the grid-connected control mode.

[0126] Figure 3 This diagram illustrates the voltage drop waveform of each VSC before mode switching; the reference voltage amplitude is 311V, and the load power is 80kW. Before 0.5s, the grid-connected voltage amplitudes of VSC1, VSC2, and VSC3 are 0.934V.pu, 0.930V.pu, and 0.926V.pu, respectively, meeting the ±10% requirement for distribution network node voltage amplitude deviation. Between 0.5s and 0.52s, a 40kW power disturbance occurs in the load, returning to normal after 0.52s. Within this range, the system stability margin is insufficient, and the grid-connected voltage amplitudes of VSC1, VSC2, and VSC3 drop to a maximum of 0.89V.pu, 0.884V.pu, and 0.878V.pu, respectively, failing to meet the ±10% requirement for distribution network node voltage deviation.

[0127] Figure 4This diagram illustrates the voltage drop waveform of each VSC after mode switching. The reference voltage amplitude is 311V, and the load power remains 80kW. VSC1 switches to VSG network control mode. Within 0 to 0.5s, the grid-connected voltage amplitudes of VSC1, VSC2, and VSC3 are 0.934V.pu, 0.930V.pu, and 0.926V.pu, respectively, meeting the ±10% requirement for distribution network node voltage amplitude deviation. Within 0.5s to 0.52s, a 40kW power disturbance occurs in the load, which recovers to normal after 0.52s. Within 0.5-0.52s, the system has sufficient stability margin, and the grid-connected voltage amplitudes of VSC1, VSC2, and VSC3 drop to a maximum of 0.920V.pu, 0.915V.pu, and 0.910V.pu, respectively, meeting the ±10% requirement for distribution network system node voltage deviation.

[0128] According to the inverter control mode switching method proposed in the multi-inverter access distribution network system of the present invention, when the stability margin of the distribution network system is insufficient, there is a risk of voltage exceeding the limit under load disturbance. Switching VSC1 to network mode control can ensure that the node voltage deviation meets the requirements of the distribution network system under load disturbance, thus ensuring the stable operation of the distribution network system.

Claims

1. A method for switching inverter control modes in a distribution network system with multiple inverters, comprising the following steps: S1. Obtain the system parameters of the target distribution network system; S2. Based on the system parameters obtained in step S1, establish the equivalent output admittance of each grid-connected control mode inverter and each grid-connected control mode inverter respectively; S3. Based on the admittance data obtained in step S2, calculate the equivalent circuit model of the target distribution network system in the form of full admittance. S4. Calculate the determination phase value between the target distribution network system and the circuit model obtained in step S3; S5. Based on the determined phase value obtained in step S4, calculate the normalized phase angle stability margin of the grid-connected system; S6. Determine the normalized phase angle stability margin of the grid-connected system obtained in step S5: If the normalized phase angle stability margin is less than the set first threshold, then select the optimal inverter among the grid-following control mode inverters and convert it to a grid-connected control mode inverter. If the normalized phase angle stability margin is greater than or equal to the first threshold and less than or equal to the second threshold, then the current control mode of all inverters remains unchanged. If the normalized phase angle stability margin is greater than the set second threshold, then select the optimal inverter among the grid-connected control mode inverters and convert it to a grid-connected control mode inverter. S7. Repeat steps S2 to S6 in real time to complete the inverter control mode switching in a distribution network system with multiple inverters connected.

2. The inverter control mode switching method in a distribution network system with multiple inverters as described in claim 1, characterized in that... Step S1, which involves obtaining the system parameters of the target distribution network system, specifically includes the following steps: The target distribution network system includes grid power supply, three-phase load, N grid-connected control mode inverters and M grid-connected control mode inverters; N and M are both natural numbers; The acquired system parameters include grid admittance Y. g .

3. The inverter control mode switching method in a distribution network system with multiple inverters as described in claim 2, characterized in that... Step S2, which involves establishing the equivalent output admittance of each grid-connected control mode inverter and each grid-connected control mode inverter based on the system parameters obtained in step S1, specifically includes the following steps: Using a harmonic linearized impedance modeling method, the equivalent output admittance Y of N grid-fed control mode inverters is established. eq_1 ,Y eq_2 ,...,Y eq_N ;Y eq_i Let be the equivalent output admittance of the i-th grid-controlled inverter; Using a harmonic linearized impedance modeling method, the equivalent output admittance Y of M grid-controlled inverters is established. cq_1 ,Y cq_2 ,...,Y cq_M ;Y cq_j Let be the equivalent output admittance of the j-th grid-controlled inverter.

4. The inverter control mode switching method in a distribution network system with multiple inverters access according to claim 3, characterized in that... Step S3 involves calculating the equivalent circuit model of the target distribution network system in the form of full admittance, which specifically includes the following steps: Based on the equivalent output admittance of each inverter obtained in step S2, the equivalent circuit model of the target distribution network system in the form of full admittance is obtained. The equivalent output admittance of each inverter is simplified to the output admittance Y using the following formula. eq : In the formula Y eq_i Y is the equivalent output admittance of the i-th grid-controlled inverter; cq_j Let be the equivalent output admittance of the j-th grid-controlled inverter.

5. The inverter control mode switching method in a distribution network system with multiple inverters access according to claim 4, characterized in that... Step S4 involves calculating the phase value between the target distribution network system and the circuit model obtained in step S3, specifically including the following steps: Plot the output admittance Y eq With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve; Based on the plotted curve, obtain Y. eq and Y g The intersection frequency corresponding to the intersection point of the amplitude-frequency response curve; In Y eq and Y g On the phase frequency response curve, obtain the output admittance phase value θ1(Y) corresponding to the intersection frequency. eq ) and grid admittance phase value θ1(Y g ).

6. The inverter control mode switching method in a distribution network system with multiple inverters as described in claim 5, characterized in that... Step S5, which involves calculating the normalized phase angle stability margin of the grid-connected system based on the determined phase value obtained in step S4, specifically includes the following steps: The normalized phase angle stability margin of the grid-connected system is calculated using the following formula: In the formula, PM1 is the normalized phase angle stability margin of the grid-connected system.

7. The inverter control mode switching method in a distribution network system with multiple inverters as described in claim 6, characterized in that... If the normalized phase angle stability margin is less than a set first threshold, step S6 involves selecting the optimal inverter from the grid-connected control mode inverters and converting it to a grid-connected control mode inverter. This specifically includes the following steps: If the normalized phase angle stability margin is less than the set first threshold, then the following steps A to G are used to calculate the stability margin for each grid-connected control mode inverter in sequence: A. Using the impedance modeling method with harmonic linearization, the equivalent output admittance Y when the k-th grid-controlled inverter switches to grid-connected control mode independently is calculated. cq_k ; B. Establish an equivalent circuit model in full admittance form after mode switching, and use the following formula to convert the equivalent output admittance of all inverters into the second output admittance Y. cq : C. Plot the second output admittance Y cq With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve; based on the plotted curves, obtain the Y... cq and Y g The second intersection frequency corresponding to the intersection point of the amplitude-frequency response curves; in Y cq and Y g On the phase frequency response curve, obtain the second output admittance phase value θ2(Y) corresponding to the second intersection frequency. cq ) and the second grid admittance phase value θ2(Y g ); D. Calculate the second normalized phase angle stability margin of the grid-connected system at this time: In the formula, PM2 is the second normalized phase angle stability margin of the grid-connected system; E. By calculating the power flow, the voltage U of each node in the target distribution network system at this time is obtained. x ; F. Calculate the average node voltage deviation ΔU of the target distribution network system at this time: In the formula, n is the number of nodes in the target distribution network system; U x_ref The nominal voltage of the node; G. Calculate the evaluation index Q for the grid control mode switching of the kth grid-controlled inverter. k For Q k =k1*PM2-k2*ΔU; where k1 and k2 are both set weighting coefficients; Finally, the inverter with the highest grid-following control mode evaluation index was selected, and the inverter was switched to grid-connected control mode and put into operation.

8. The inverter control mode switching method in a distribution network system with multiple inverters access according to claim 6, characterized in that... If the normalized phase angle stability margin is greater than the set second threshold, then in step S6, an optimal inverter is selected from the grid-controlled inverters and converted to a grid-following control mode inverter. This specifically includes the following steps: If the normalized phase angle stability margin is greater than the set second threshold, then steps a to g are performed sequentially to calculate the stability margin for each grid-controlled inverter: a. Using the impedance modeling method with harmonic linearization, the equivalent output admittance Y when the k-th grid-controlled inverter switches to grid-following control mode independently is calculated. eq_k ; b. Establish an equivalent circuit model in full admittance form after mode switching, and use the following formula to convert the equivalent output admittance of all inverters into the third output admittance Y. eq2 : c. Plot the third output admittance Y eq2 With grid admittance Y g The amplitude-frequency response curve and phase-frequency response curve; based on the plotted curves, obtain the Y... eq2 and Y g The frequency of the third intersection point corresponding to the intersection of the amplitude-frequency response curves; in Y eq2 and Y g On the phase frequency response curve, obtain the third output admittance phase value θ3(Y) corresponding to the third intersection frequency. eq2 ) and the second grid admittance phase value θ3(Y g ); d. Calculate the third normalized phase angle stability margin of the grid-connected system at this time: In the formula, PM3 is the third normalized phase angle stability margin of the grid-connected system; e. Through power flow calculation, obtain the voltage U of each node in the target distribution network system at this time. x ; f. Calculate the average node voltage deviation ΔU of the target distribution network system at this time: In the formula, n is the number of nodes in the target distribution network system; U x_ref The nominal voltage of the node; g. Calculate the grid-following control mode switching evaluation index Q' of the k-th grid-controlled inverter. k For Q' k =k3*PM3-k4*ΔU; where k3 and k4 are both set weighting coefficients; Finally, the inverter with the highest grid-following control mode switching evaluation index was selected, and the inverter was switched to grid-following control mode and put into operation.

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